This disclosure relates to an electrode for capacitive deionization device, and too a method of manufacturing the electrode and a capacitive deionization device that comprises the electrode.
Recently, rapid industrialization has inflicted damage to the environment and has induced climatic change. Thus, clean water has been drying up, while demand for purified water has been increasing due to increases in human population and improvements in the standard of living. In addition, a lot of attention has been paid to develop a method of purifying water.
Water softener, which converts hard water to soft water acts to reduce the amount of cations in water to less than a predetermined level. In general, ion exchange resins have been used in water softening systems to soften water. In such systems, cations (such as bivalent ions) present in the hard water are removed by being exchanged with Na+ ions adsorbed to the ion exchange resin. The Na+ ions are thus released into the water while the bivalent ions are adsorbed into the ion exchange resin. When the ion exchange resin is saturated with the bivalent ions and the capability of the resin for further exchange of ions is reduced, the resin is restored through a reversible reaction by bringing NaCl into contact with it. The bivalent ions adsorbed to the resin are desorbed through a concentration differential by contacting it with a large amount of NaCl, and Na+ is adsorbed to the ion exchange resin again. The ion exchange resin is widely used in water softener systems and is easily available.
However, the ion exchange resin has a serious drawback with regard to secondary waste, since about 100 pounds of NaCl is required to restore 1 pound of the ion exchange resin. In addition, water generated during the regeneration of the ion exchange resin contains a large amount of Cl− ions, thereby being harmful to plants. Accordingly, damage to crops by the waste water containing Cl− ions is often reported in farms across the United States, and the costs for treating Na+ ions is therefore increasing. Thus, the development of technology replacing such ion exchange resins is urgently required to inhibit such environmental damage.
Capacitive Deionization (CDI) is a system in which ionic materials in a medium are adsorbed on the surface of an electrode and then removed by applying a voltage to a nano porous carbon electrode so as to provide the electrode with a polarity. The adsorbed ionic materials are discharged with water by applying an inverse voltage to the electrode during its regeneration. This system has similar characteristics to conventional supercapacitors. CDI is advantageous since chemicals are not required for regeneration, and ion exchange resins, expensive filters or membranes are not required. Elements of hard water and harmful ions are removed and capacitance increases in CDI.
In CDI, when a direct current having a low potential difference is applied to water flowing between the carbon electrodes in which ions are dissolved, anions among the dissolved ions are adsorbed on the cathode and cations are adsorbed on the anode. When the application of the direct current ceases, the concentrated ions are desorbed from the electrodes and removed. Here, low resistance and wide specific surface area are desirable properties for the carbon electrodes. Thus, the carbon electrode may be prepared by binding active carbon using a polytetrafluoroethylene (PTFE) binder, or by forming a plate through carbonization of a resorcinol formaldehyde resin and complicated drying process (U.S. Pat. Nos. 5,196,115 and 5,425,858).
However, the electrode using the active carbon and the PTFE binder needs to be prepared from a paste, and thus operational efficiency may decrease, and electrical conductivity may decrease since a large amount of the binder is required.
Although the electrode prepared from resorcinol formaldehyde resin has high efficiency since it has low electric resistance and wide specific surface area, the preparation process is very complicated, and thus manufacturing costs, working load, and maintenance expenses are high.